temperatures are generally lower than the melting points of both base metals. In addition, very fast heating and cooling rates can be used during the brazing process to minimize the thickness of intermetallic compounds that might form along the interfaces (Ref. 10). The benefits of using laser brazing and laser welding-brazing technologies for joining dissimilar materials are also becoming increasingly recognized due to the combined attributes of furnace brazing and laser welding (Ref. 11). With a more localized energy input and more precise control of the laser beam energy, high joining speeds and accompanying high cooling rates can be realized with minimal heating of the parts. Also, laser brazing and laser welding-brazing can prevent or minimize excessive formation of detrimental intermetallic phases. If intermetallic layers can be limited to thicknesses below 10 μm, acceptable joint strengths and mechanical properties may be obtained (Refs. 12–14). In our previous study (Ref. 15), a diode laser brazing process was developed for joining Mg alloy sheet to aluminized steel sheet where the Al-12Si coating served as the interlayer. This coating was found to promote wetting of the steel by the magnesium brazing alloy; however, a preexisting layer of brittle θ-FeAl3 along the braze-steel interface was found to degrade the mechanical properties of the joint as failure of the joint always occurred by fracture of this brittle intermetallic layer. Following a review of binary and ternary phase diagrams, nickel was identified as a potentially viable interlayer element between the steel and Mg-9Al-2Zn brazing alloy used. Therefore, the purpose of this present study was to investigate the brazeability, interfacial microstructure, and mechanical properties of the laser brazed AZ31B-H24 magnesium alloy to steel sheet with an electrodeposited layer of Ni on the steel to act as the interlayer element. It is expected that development of this laser brazing technology for joining of steel-interlayer-Mg alloy combinations with a strong metallurgical bond between the steel and Mg alloy will facilitate increased application and use of Mg alloys in the automotive industry. Experimental Procedure In this study, 2-mm-thick commercialgrade twin-roll strip cast AZ31B-H24 Mg alloy sheet and 1-mm-thick steel sheet were used as the base materials. The chemical compositions of the base materials are given in Tables 1 and 2. A 2.4-mmdiameter TiBraze Mg 600 filler metal (Mg-Al-Zn alloy) with solidus and liquidus temperatures of 445° and 600°C, respectively, was chosen for this study. The commercial flux used in the experiments was Superior No. 21 manufactured by Superior Flux and Manufacturing Co. This powder flux was composed of LiCl (35–40 wt-%), KCl (30–35 wt-%), NaF (10–25 wt- %), NaCl (8–13 wt-%), and ZnCl2 (6–10 wt-%) (Ref. 16). The AZ31B Mg and steel sheets were cut into 60- × 50-mm specimens. Prior to laser brazing, the oxide layers on the surfaces of the magnesium sheets were removed by stainless steel wire brushing. All the specimens were ultrasonically cleaned in acetone to remove oil and other contaminants from the specimen surfaces. The edge of each steel sheet was bent in order to make a single-flare bevel lap joint after clamping against the magnesium sheet. After bending, the steel specimens were cleaned in acetone and then ground to 1000 grit using SiC abrasive paper and again ultrasonically cleaned in acetone. The prepared surfaces were then immediately electroplated with electrolytic pure nickel. In the Ni electroplating process, the clean steel sample was the cathode and graphite was the anode. The composition of the electroplating solution and the electroplating conditions are listed in Table 3. Figure 1A shows a schematic of the Ni electrodeposition process used. In order to get a uniform 5- μm-thick Ni layer on the steel, different cathode current densities and plating times were tested. Electrodeposition of Ni using a cathode current density of 120 mA/cm2 for 10 min was found to provide a JANUARY 2013, VOL. 92 2-s WELDING RESEARCH Fig. 1— A — Schematic of the Ni electrodeposition process on steel; B — transverse section of the Ni electrodeposited layer on the steel substrate. Table 1 — Measured Chemical Composition of the AZ31-H24 Mg Alloy Sheet and TiBraze Mg 600 Filler Metal (wt-%) Al Zn Mn Si Mg AZ31B-H24 3.02 0.80 0.30 0.01 Bal. TiBraze Mg 600 9.05 1.80 0.18 — Bal. Table 2 — Measured Chemical Composition of the 1-mm-Thick Steel Sheet (wt-%) C 0.01 Mn 0.5 P 0.010 S 0.005 Fe Bal. A B
Welding Journal | January 2013
To see the actual publication please follow the link above